1. Field of the Invention
The present invention relates to a stretched film of polypropylene which is excellent in transparency and which does not suffer from problems that upon winding films on a roll over and over, such as the films are adhered to one another which makes it difficult to unroll the film and uneven dimensional change due to sliding of the rolled film with time.
2. Description of the Prior Art
Polypropylene is not expensive and has relatively high rigidity. In addition, it has good transparency even when it is formed into a stretched film. For this reason, polypropylene has widely been used in the form of stretched films in various uses. When polypropylene is in general used in the form of stretched films, the stretched film is wound on a roll, stored in the rolled state and unrolled when it is put in practical use. However, unrolling of the stretched film is accompanied by various problems such as difficulty of unrolling and twisting of the film. To solve such problems, an antiblocking agent (hereunder referred to as "AB agent") is commonly added to the polypropylene films. Examples of such AB agents are fine particles of inorganic compounds such as silica, cross-linked polysiloxanes and melamineformaldehyde condensates.
The foregoing method in which an AB agent is incorporated into the polypropylene films is effective for preventing blocking, but suffers from various problems. For instance, the transparency of the resulting stretched film becomes insufficient since voids are formed at the portions in which particles of the AB agents used are present when such a polypropylene film is stretched. It is difficult to uniformly disperse the AB agents, blocking is thus caused at the portions where the particles of the AB agents are absent, this in turn leads to non-uniform change in the rolled state with time. Thus, dimensional change of the film also becomes non-uniform and consequently discrepancy in printing during post processing can occur.